Method of fractionating natural gas feed by preheating feed with fractionator overhead



Jan- 9, 1968 w. D. A. BURNS ETAL 3,362,175

METHOD OF' FRACTIONATING NATURAL GAS FEED EY PREHEATING FEED WITHFRACTIONATOR OVERHEAD Filed July '7. 1965 N Oa of c aL .E o gg g g a...C E l Q g INVENToRs D William D. A. Burns Geoffrey M. Rowell ManuelSonia Olalla ATTORNEY United States Patent O ABSTRACT F THE DISCLOSURE Amethod of separating the heavier hydrocarbons from liquefied natural gasin a fractionating zone which includes introducing into the upper partof the fractionating zone hydrocarbons heavier than C2 which have beencooled by heat exchange with liquid natural gas being fed to thefractionating zone.

This invention relates to a method of separating the heavierhydrocarbons from liquefied natural gas containing a substantialproportion of methane.

A method of this kind is the subject of British patent specification No.854,099, corresponding to U.S. Patent No. 2,952,984, in which the methodclaimed comprises the steps of:

(a) Feeding the liquefied natural gas at superatmospheric pressure intothe medial portion of a fractionating zone;

(b) Heating the contents of the lower portion of the fractionating zoneto a temperature to produce methane enriched vapors in the upper portionof the fractionating zone;

(c) withdrawing vapors from the upper portion of the fractionating zoneand passing said vapors in heat exchange relation with the liquefiednatural gas being fed to the fractionating Zone, to heat said feed andcool said vapors;

(d) Separating condensates from said vapors;

(e) Refiuxing said condensates to the upper portion of the fractionatingZone for cooling the upper portion of the fractionating zone andinducing downward movement of heavier hydrocarbon enriched liquefied gasinto the lower portion of the fractionating zone; and

(f) Withdrawing the heavier hydrocarbon enriched liquefied gas from thelower portion of the fractionating zone.

In the above method the heavier hydrocarbons and the methane areseparated from one another in an efficient manner by making use of thecold which is available in the liquefied natural gas, whilesimultaneously the liquefied natural gas is regasified for its furtheruse after having served as a refrigerant.

The present invention relates to an improvement of the method of Britishpatent specification No. 854,099, whereby a larger proportion of themethane and a smaller proportion of the heavier hydrocarbons supplied tothe fractionating Zone in the liquefied natural gas feed may bewithdrawn from the top of the fractionating zone, and correspondingly, asmaller proportion of the methane and a larger proportion of the heavierhydrocarbons originally in the liquefied natural gas feed leave thefractionating Zone as a bottom product.

According to the present invention, the above method of separating theheavier hydrocarbons from liquefied natural gas containing a substantialproportion of methane comprises the additional step of introducing a C3rice and/or heavier hydrocarbon material into the upper portion of thefractionating zone.

The method of the invention will be explained further by way of examplewith reference to the accompanying drawing, which represents a flowscheme for this method.

In the drawing the liquefied natural gas is supplied to the medial zoneof a fractionator or demethaniser 19 through line 11. The liquefiednatural gas before being introduced into line 11 has already beenallowed to flash off some of its lighter components, and the compositionof the liquefied natural gas which is fed to the demethaniser 10 is asfollows:

It Will be understood that this composition is an example only and thatthe liquefied natural gas which is processed in the demethaniser 16 mayhave many different compositions.

The liquefied gas is introduced into line 11 at a temperature of F. anda pressure of about 210 p.s.i.a. Upon passing through a heat exchanger14 the temperature of the liquefied gas rises to 70 F., and afterpassing through another heat exchanger 15, the temperature has increasedto 55 F. At this temperature of 55 F. the liquefied gas is introduced asfeed to the medial Zone of the demethaniser 10. In the demethaniser 10,which operates at p.s.i.a., the feed is heated and thereby separatedinto a top fraction consisting of methane enriched vapors and a bottomproduct containing the heavier hydrocarbons. The bottom temperature ofthe demethaniser 10 is maintained at 54 F. The reboiler circuit used forheating the demethaniser 10 is shown at 19a. Valve controls for thebottom products of columns 29 and 12 are shown at 26a and 28arespectively. All pumps, valves, etc., not necessary to understand themethod of the invention have been left out of the drawing.

The composition of the vapors leaving the top of the demethaniser 10 isas follows:

Attention is drawn to the fact that the quantity of methane in theseoverhead vapors is 99.7% of the quantity of methane in the feed to thedemethaniser 10. The vapors are withdrawn from the demethaniser 10through line 17. Before the vapors subsequently pass in heat exchangewith the liquefied natural gas in heat exchanger 14 they are joined witha stream of C4 hydrocarbons being at a temperature of 30 F. and flowingto line 17 through line 23. The combined stream of methane enrichedvapors and C4 hydrocarbons now passes through the heat exchanger 14,whereby the vapors are cooled and partly condensed. The condensates fromthe vapors are collected in a reliux vessel 18, from where thecondensates together with the C4 hydrocarbons are conducted to the topof the demethaniser 10 through line 19. In

3 the demethaniser the C4 hydrocarbons serve as a refrigerant to coolthe top of the demethaniser and as an absorption liquid to remove the C2and heavier hydrocarbons from the vapors leaving the demethaniser 10.

It will be understood that the C4 hydrocarbons could have beenintroduced immediately into the top of the demethaniser 10 through line23a, or into line 17 between the heat exchanger 14 and the redux vessel18 through line 231) if the temperature of the C4 hydrocarbons in line23 had already been at a desired level. However, the introduction intoline 17 upstream of the heat exchanger 14 is normally to be preferred.

The temperature of the liquids collected in the reiiux vessel 18 andpassed to the demethaniser 10 is about 30 F. to maintain a temperatureof -20 F. in the top portion of the demethaniser 10.

The methane enriched vapors separated from the condensates in the refluxvessel 18 leaves the top of the reliux vessel 18 through line 2t! andpass through a series of heat exchangers 21 to usefully abstract heatfrom these vapors which are being warmed up from -30 F. to a temperatureof about 60 F. The methane vapors, which are at about 157 p.s.i.a., arenow discharged from the system and may be distributed as fuel gas. Theproduct formed in the bottom of the demethaniser 10 is drawn off andpassed through line 22 as a feed to a second fractionator 29, whichoperates as a de-ethaniser.

The composition of the bottom product of the demethaniser 10 is asfollows:

Thus, the quantity of methane retained in the bottom product is only0.3% of the total quantity of methane supplied to the demethaniser 10.

The de-ethaniser 29 is maintained at a pressure of 170 p.s.i.a. and at abottom temperature of about 120 F. Under these conditions the feed ofthe de-ethaniser 29 is fractionated into a top fraction which consistsmainly of C2 hydrocarbons and a bottom fraction containing C3 andheavier hydrocarbons. The C2 hydrocarbons are withdrawn from thede-ethaniser 29 through line 24 and pass on to a cracking plant 13.

The bottom fraction is drawn off from the de-ethaniser 29 through line26 and is fed to still another fractionator 12, which acts as adepropaniser. The depropaniser 12 operates under 188 p.s.i.a. and at abottom temperature of 200 F. The top fraction which consistssubstantially of C3 hydrocarbons passes from the depropaniser 12 throughline 25 to the cracking plant 13. Likewise, a portion of the bottomfraction of substantially C4 hydrocarbons is drawn o through line 27also to the cracking plant 13. In the cracking plant 13 the hydrocarbonssupplied thereto through lines 24, 25 and 27 are thermally cracked,quenched, compressed, washed and hydrogenated in a conventional mannerto produce a mixture of gases which can be separated into a number ofindividual products, including further methane gas. The other portion ofthe C4 hydrocarbons leaving the bottom of the depropaniser 12 is notdelivered to the cracking plant 13 but passes through line 28 and a heatexchanger 16. in the heat exchanger 16 the C4 hydrocarbons fiowingtherethrough are cooled by cooling water to a temperature of 100 F. TheC4 hydrocarbons are subsequently passed through heat exchanger 1S inheat exchange with the liquefied natural gas flowing through line 11,whereby the temperature of the C4 hydrocarbons is further lowi ered toabout -30 F. At this temperature the C4 hydrocarbons are dischargedthrough line 23 into line 17.

As explained, a portion of the C4 hydrocarbons drawn off from the bottomof the depropaniser 12 is sent to the top of the demethaniser 10, whilethe remaining portion passes to the cracking plant 13. instead, all ofthe C4 hydrocarbons may be conducted to the cracking plant 13, and aportion of the bottom product from the deethaniser 2@ consisting of C3and C4 hydrocarbons may be drawn off from line 26 and passed into line2S through valve 26a to be cooled in the heat exchangers 16 and 15subsequently to be discharged into line 17 for introduction into thedemethaniser 10. Alternatively, in certain circumstances, it may bepreferred to separate C2 and C3 hydrocarbons together as a top productin the second fractionator 29, C4 hydrocarbons being produced as abottom product which is either passed to the demethaniser 10 or crackedin the cracking plant 13. The C2 and C3 hydrocarbon mixture would eitherbe separated by subsequent fractionation or be cracked. Stillalternatively, C3 or C5 hydrocarbons alone or a mixture of hydrocarbonsheavier than C2 hydrocarbons may be introduced into the demethaniser 10,or the hydrocarbons to be administered to the demethaniscr 10 may besupplied by sources other than the de-ethaniser 29 or the depropaniser12. For example, C3 and/or C4 hydrocarbons may be separated from themixture of gases produced in the cracking plant 13 and, after havingbeen brought at the proper temperature and pressure level, be suppliedwholly or partly to the demethaniser 10.

It will be apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementwithin the scope of our invention as defined in the appended claims.

We claim:

1. In a method of separating the heavier hydrocarbons from liquefiednatural gas containing a substantial proportion of methane, the stepsof:

(a) feeding the liquefied natural gas at superatmospherie pressure intothe medial portion of a fractionating zone;

(b) heating the contents of the lower portions of the fractionating Zoneto a temperature to produce methane enriched vapours in the upperportion of the fractionating zone;

(c) withdrawing the vapours from the upper portion of the fractionatingzone and passing said vapours in heat exchange relation with theliquefied natural gas being fed to the fractionating zone, to heat saidfeed and cool said vapours;

(d) separating eondensates from said vapours;

(e) refiuxing said eondensates to the upper portion of the fractionatingzone and inducing downward movement of heavier hydrocarbon enrichedliquefied gas into the lower portion of the fractionating zone;

(f) introducing at least one heavier hydrocarbon material heavier thanC2 into the upper portion of the fractionating zone; and

(g) withdrawing the heavier hydrocarbon enriched liquefied gas from thelower portion of the fractionating zone;

(h) passing the said hydrocarbon material before introduction into thefractionating zone in heat exchange with the liquefied natural gas beingfed to the fractionating zone, to heat the liquefied natural gas and tocool the hydrocarbon material.

2. A method according to claim 1, wherein the liquelied natural gasbeing fed to the fractionating zone is passed in heat exchange with thesaid hydrocarbon material after having passed in heat exchange with thevapours withdrawn from the top of the fractionatng zone.

3. A method according to claim 2, wherein the said hydrocarbon materialis introduced into the reflux eircuit of the fractionating zone throughwhich the methane enriched vapours are withdrawn from, and thecondensates from these vapours are returned to, the fractionating zone.

4. A method according to claim 3, wherein the said hydrocarbon materialis discharged into the reflux circuit at a point before the methaneenriched vapours pass in heat exchange with the liquefied natural gasbeing fed to the fractionating zone.

5. A method according to claim 4, wherein the heavier hydrocarbonenriched liquefied gas from the lower portion of the fractionating zoneis fed to at least one furlo upper portion of the fractionatng zoneproducing the methane enriched vapours.

References Cited UNITED STATES PATENTS 2,775,103 12/1956 Koble et a162-17 2,804,488 8/1957 Cobb 62-17 X 2,952,984 9/ 1960 Marshall 62-273,262,278 7/ 1966 Thorsten et al. 62--17 X NORMAN YUDKOFF, PrimaryExaminer.

V. W. PRETKA, Assistant Examiner.

